The present invention relates to a fuel pump control circuit and, more particularly, to an improvement in the invention of U.S. Ser. No. 07/598033 filed by the present applicants to the United States of America on Oct. 16, 1990.
The fuel pump control circuit of the above invention employs an SCR (thyristor: tradename). Upon turning on of an ignition switch, a gate current is supplied to the SCR via a first conduction path, thereby turning on the SCR. Consequently, a power source voltage is supplied to a fuel pump circuit for driving a fuel pump. When a predetermined time elapses in a timer circuit after the ON of the ignition switch, the gate current flowing into the first conduction path is shunted to a second conduction path. Since the gate current is decreased by this shunting and this decreases an anode current to be smaller than a holding current, the SCR is turned off to stop the supply of the power source voltage to the fuel pump circuit.
According to this fuel pump control circuit, therefore, a fuel is supplied to a carburetor for a predetermined time period immediately after turning on of the ignition switch. For this reason, even if a carburetor or piping of a vehicle is empty because the vehicle is a new one or it is left unused for a long time period or at a high temperature under a blazing sun, the carburetor or the piping is filled with a fuel upon start of an engine, and this improves the starting properties of the engine.
When, however, the above fuel pump control circuit is used in a high-temperature area or is mounted on a portion close to an engine of an actual car, the temperature of the SCR is sometimes increased abnormally due to heat derived from its location together with heat generated by its own circuit elements. In this case, a gate current value (to be referred to as a turn-on minimum current value hereinafter) by which the SCR can be turned on decreases to possibly prevent turning off of the SCR even when a predetermined time elapses in the timer circuit.
That is, the gate current flowing into the first conduction path abruptly decreases upon shunting into the second conduction path. Since, however, the turn-on minimum current value of the SCR decreases at an abnormally high temperature, the inflow gate current becomes larger than this turn-on minimum current value. For this reason, the SCR is not turned off even when the anode current is smaller than the holding current, and this makes it impossible to stop supply of a fuel to a carburetor.
Also, in the above fuel pump control circuit, if an abnormal voltage rise is caused by, e.g., a failure in a regulator, the gate current flowing into the SCR is increased to possibly prevent turning off of the SCR even when a predetermined time elapses in the timer circuit.
That is, when an abnormal voltage rise occurs, the gate current flowing into the first conduction path upon shunting into the second conduction path increases to be larger than the turn-on minimum current value. Therefore, by the same mechanism as in the case of an abnormally high temperature, the SCR is not turned off to fail to stop supply of a fuel to a carburetor.
Note that in order to solve the above problems, it may be possible to use an SCR having specifically selected temperature characteristics or to additionally provide a constant-current circuit. This, however, results in a high cost.